Dmitri Mendeleev

What is a mark of a great scientist? Good scientists
discover new information and make sense of it, linking it to other data.
They may go further by giving an explanation of this linked data which,
maybe not immediately, other scientists accept as a correct explanation.
However the outstanding scientist goes further in predicting consequences
of his ideas which can be tested. This boldness identifies the great
scientist if the predictions are later found to be accurate. One such
person was Russian chemist
Dmitri Mendeleev. Incidentally, although he is often regarded as the
father of the Periodic Table, Mendeleev himself called his table, or
matrix, the Periodic System.

Formulator of the Periodic Table

A commemorative stamp collector’s miniature
sheet showing some of Mendeleev’s original notes. Horizontal lines
like Cr, Mo and W (in the third row down) correspond to today's
groups. Note the date, 17 February 1869.

Other people, like Londoner
John Newlands, Frenchman
Alexandre Béguyer de Chancourtois and German
Julius Lothar Meyer made important contributions to the first Periodic
Table but the main credit goes to Mendeleev. All of them were helped by
the publication in 1860 of more accurate atomic weights, as relative
atomic masses were then called. There were two main problems about
establishing a pattern for the elements. First only 60 elements had been
discovered (we now know of over 100) and second some of the information
about the 60 was wrong. It was if Mendeleev was doing a jigsaw with one
third of the pieces missing, and other pieces bent!

Mendeleev had written the properties of elements on
pieces of card and tradition has it that after organising the cards while
playing patience he suddenly realised that by arranging the element cards
in order of increasing atomic weight that certain types of element
regularly occurred. For example a reactive non-metal was directly followed
by a very reactive light metal, then a less reactive light metal. The
image of a stamp collectors’ miniature sheet shows a stamp commemorating
the hundredth anniversary of the Periodic Table superimposed on some of
Mendeleev’s original jottings.

Shortly after, his ideas were presented to the
Russian Physico-chemical Society. They were read by Professor Menschutkin
because Mendeleev was ill. His ideas were then published in the main
German chemistry periodical of the time, Zeitschrift fϋr Chemie.

The world’s first view of Mendeleev’s Periodic
Table – an extract from Zeitschrift fϋr Chemie, 1869. Click here
for a translation

What were the special features of Mendeleev's Periodic Table?

Why is Mendeleev considered to be the ‘father’ of the
Periodic Table whilst others, such as Newlands, Meyer and De Chantcourtois
are considered to be also-rans?

First he put elements into their correct places in
the table. In some cases the relative atomic mass had been wrongly
calculated by others. By correcting the relative atomic mass he put the
element in the correct place.

At the time, relative atomic masses (then called
atomic weights) were laboriously determined using the formula

atomic weight = equivalent weight x valency

The combining (or equivalent) weights (see Problems with
relative atomic masses) were generally accurate but sometimes an
element was given the wrong valency. Thus beryllium, combining weight 4.6,
was given the valency 3 because it was chemically similar to aluminium.
This gave an atomic weight of 13.8, placing it between carbon and nitrogen
where there was no space. Mendeleev said the valency was 2; the problem
was solved - it fitted into the space between lithium and boron.

Secondly, Mendeleev sometimes decided that atomic
weights must be wrong because the elements simply appeared in the wrong
place. For example he placed tellurium before iodine although its atomic
weight is greater simply because iodine’s properties are so similar to
those of fluorine, chlorine and bromine and tellurium’s to those of
oxygen, sulfur and selenium rather than the other way round. We now know
that it is atomic number, not relative atomic mass that governs an
element’s position in the Periodic Table but in most cases the two result
in the same order.

Correct predictions

The greatness of Mendeleev was that not only did he
leave spaces for elements that were not yet discovered but he predicted
properties of five of these elements and their compounds. How foolish he
would have seemed if these predictions had been incorrect but fortunately
for him three of these missing elements were discovered by others within
15 years (ie within his lifetime). The first of these Mendeleev had
called eka-aluminium because it was the one after aluminium (eka = 1 in
Sanskrit) and was identified in Paris (1875) by Paul Emile Lecoq de
Boisbaudran who named it gallium after the Latin name for France.
Mendeleev was ecstatic when he heard of its properties which nearly
matched his eka-aluminium. However de Boisbaudran's value for gallium's
density (4.9 g/cm3) differed from Mendeleev's prediction.
Mendeleev told the Frenchman, who re-measured the density to find
Mendeleev was right! It is interesting to speculate whether de Boisbaudran
was pleased or irritated by this. The table compares Mendeleev's
predictions with de Boisbaudran's discovery.

Eka-aluminium (Ea)

Gallium (Ga)

Atomic weight

About 68

69.72

Density of solid

6.0 g/cm3

5.9 g/cm3

Melting point

low

29.78oC

Valency

3

3

Method of discovery

Probably from its spectrum

Spectroscopically

Oxide

Formula Ea2O3, density
5.5 g/cm3. Soluble in both acids and alkalis.

Formula Ga2O3, density
5.88 g/cm3. Soluble in both acids and alkalis.

Within the next ten years Swede Lars Nilson (1879)
identified scandium, predicted by Mendeleev as eka-boron and German
Clemens Winkler (1886) discovered germanium which he realised was
Mendeleev's eka-silicon. These discoveries established the acceptance of
the Russian's table, although two other elements whose properties were
predicted were not discovered for 50 years.

One thing that Mendeleev did not predict was the
discovery of a whole new Group of elements, the noble gases, by the Scot
William Ramsay and co-workers during the last decade of the 19th
century (see The discovery of new
elements). Mendeleev was at first dismayed by this but before he
died in 1907 realised that Ramsay's discoveries were further proof of the
Periodic Table, not a contradiction. Ramsay was awarded a
Nobel Prize for discovering five elements. Mendeleev never received
that honour However, an element, atomic number 101, has been named after
Mendeleev, an even rarer distinction. This is surely deserved by the
original formulator of the Periodic Table.

(b) Elements in black were discovered between Mendeleev's death and
1934.

(i) How many are there?

(ii) Where are most of these elements placed in most Periodic Tables?

(c) There are still some blanks. Give the names and symbols of the
alkali metal and halogen still to be discovered.

(d) What are the normal symbols for the elements J and Jr?

(e) At the bottom of the columns are general formulae for the oxides and
hydrides of elements in that column. Look carefully and state the
difference between them and the normal way of writing their formulae.